It was mostly tongue-in-cheek humor.
There's something to the idea that a more established and, for want of a given word since I certainly wouldn't call Valyria that even if they would, "enlightened" society, would want to make sure only people fully adults do things like get married, enlist in military service, or go through potentially traumatic magical awakenings.
I think for one thing, you got kids awakening so young in this quest because the world is so dangerous and there's a lot of factors contributing to stress which might engage "bloodlines". Like for Rhaenys it's more like "I want to gain magic so I can help my family like they helped me and my mom", for Shireen it would be something similar "I need magic to become a stronger heir to my father, so I can protect my own inheritance and Get Shit Done like the rest of my extended family".
It's funny how Viserys possesses two of Robert Baratheon's bastards and isn't at all interested in turning them into child soldiers... heck, he gave Mia Stone guards to protect her, not keep her prisoner.